The invention concerns a comparator of voltages, in which input currents of input transistors, whereto said voltages are conducted, are subtracted one from another and the current difference is converted into a voltage, which is amplified and conducted to a gate terminal of a switching transistor at an input of an output stage of the voltage comparator, whereat the voltage comparator of the invention has an especially fast response to an input voltage change at one direction of a sign reversal of the difference of the input voltages.
The invention solves the technical problem how to improve the voltage comparator circuit as described at the beginning in order to have a fast response to an input voltage change at one direction of a sign reversal of the difference of the input voltages.
Each conversion of an electric signal from the analogous representation into the digital one begins by comparing a signal voltage to individual quantized voltage levels.
A voltage comparator compares which one of the voltages at its two input terminals is higher and, depending on the result of the comparison, produces a potential of a high or low supply voltage at its output terminal.
Most voltage comparators are composed of an input stage, which compares both input voltages, an amplification stage, which amplifies the small voltage difference of usually current signals enteringit, and an output stage, which generates a digitally represented output signal.
The voltage comparator must have an exactly determined position of the switch point and a sufficiently high switching rate. The rate of an individual switching is essentially determined by the time period needed by the voltage comparator after the moment of the sign reversal at the difference of the input voltages to change the logical state at its output.
A new value of the potential at the output terminal of the voltage comparator is generated as a result of voltage changes at its internal terminals, however, these voltage changes and the resulting generation of the potential at the output terminal of the voltage comparator are delayed because of parasitic capacitances and resistances in the voltage comparator circuit.
It is often important to the operation of a complex circuit that the voltage comparator assigned thereto responds rapidly to an input voltage change at only one direction of the sign reversal of the difference of the input voltages. For example, the voltage comparator must accurately detect a passage of only an increasing signal through the reference value, whereas the response rate of the voltage comparator at a decreasing signal is not important.
When a high switching rate is needed, generally a technology providing a faster response can be chosen. The technology is usually determined by requirements with respect to other circuits on the same substrate, however, the choice is also influenced by the price.
A faster rate within an available technology is reached by increasing the density of the operating electric current in the voltage comparator. Thereby the voltage changes at the elements having parasitic capacitances are reached faster. However, the current-carrying capacity of the elements at a specified supply voltage is limited. Additionally, a high consumption of electric current is not allowed in some applications.
Further, a voltage comparator is known, which besides a basic input stage provided to generate output electric currents, which are subtracted one from another, also comprises an auxiliary input stage, to the inputs whereof also the compared voltages are conducted and which through current mirrors supplies an additional electric current to the main junction of the amplification stage (U.S. Pat. No. 6,172,535). In fact, the parasitic capacitors are loaded faster, yet the current consumption of the circuit is substantially higher.
A regenerative circuit provided with an operating point presetting is often used. The voltage comparator comprises a positive feedback loop, which is usually made as a cross-connection in the amplification stage: between elements 34, 36 and 38, 40 in the circuit of U.S. Pat. No. 6,292,030 and between elements M3A and M3B in the circuit of U.S. Pat. No. 6,392,449. As soon as the voltage in the amplification stage begins to change, the cross-connection reinforces this changing and the switching is considerably accelerated, wherefor no higher electric current is needed. To ensure that after the switching the positive feedback loop would not retain the voltage comparator in the new state, which would mean that a new sign reversal of the difference of the input voltages could not change the state of the input signal, the voltage comparator is provided with a resetting circuit (transistor 42 and M4, respectively), which opens the positive feedback loop and sets the circuit back into the state of symmetry. However, the positive feedback loop immediately pushes the circuit into either output signal state depending on the state at the voltage comparator input as soon as the resetting signal is removed, e.g. at the moments when the comparison is to be carried out, for example in an analogue-to-digital converter.
The said technical problem is solved by the voltage comparator of the invention as characterized by the features of the characterizing portion of the first claim, and the variants of the embodiment are characterized by dependent claims.
The voltage comparator of the invention is distinguished in that by means of a proposed limiter of the operating voltage displacement an especially fast response to an input voltage change is achieved at one direction of the sign reversal of the difference of the input voltages. It is robust and reliable irrespective of temperature changes and changes in the technological layout.